sideNavAccordionMode not implemented in core-modular

[JohannesDoberer]

Description

The sideNavAccordionMode property is not implemented in core-modular or the UI5 library. This property controls accordion behavior for side navigation categories — when enabled, only one category can be open at a time (expanding one collapses the others).

Expected behavior

When sideNavAccordionMode: true is set (either on a node or globally via navigation.defaults), expanding a category should automatically collapse all other expanded categories in the same side navigation level.

Current behavior

The property is not recognized. All categories can be independently expanded/collapsed (default UI5 web component behavior).

[JohannesDoberer]

Implementation hint

UI5 web components don't offer a native accordion mode on ui5-side-navigation. Each ui5-side-navigation-item / ui5-side-navigation-group manages its expanded state independently and doesn't fire a custom event on toggle.

However, LeftNav.svelte already has a MutationObserver watching for expanded attribute changes (used for persisting state to localStorage). The accordion logic could be added there:

// inside the MutationObserver callback, after storeExpandedState(...)
if (sideNavAccordionMode && target.hasAttribute('expanded')) {
  sidenav.querySelectorAll('[category-uid][expanded]').forEach(el => {
    if (el !== target) el.removeAttribute('expanded');
  });
}

The flag (sideNavAccordionMode) would need to come from leftNavData — which means core-modular needs to pass through either the node-level sideNavAccordionMode property or the global navigation.defaults.sideNavAccordionMode setting.

---

Note: This needs discussion before implementation — specifically:

• Should the accordion behavior apply per-level (only siblings) or globally across the entire side nav?
• How should it interact with the localStorage persistence (expanding one collapses others, but what happens on page reload)?
• Is this feature actually needed for the UI5 library use case, or can it be deprioritized?